Daylength regulates neuroendocrine function through mechanisms that depend on the suprachiasmatic nucleus of the hypothalamus (SCN), in which a master pacemaker resides. The SCN both generates the melatonin (MEL) rhythm critical to photoperiodic responses, and is an important target for MEL action. Nevertheless, critical questions remain about (1) the role of recently discovered clock genes in photoperiodic responses; (2) the necessity of MEL receptor expressing neurons within the SCN in seasonal responses to MEL; (3) the nature of SCN output signals which carry information about daylength to the rest of the brain. The objective of specific aim 1 is to determine whether Per1 and Per2 expression patterns within the SCN regulate the profile of the nocturnal MEL signal. Anti-sense ODNs directed against Per1 and Per2 will be reverse dialyzed within the 3rd ventricle in an attempt to modify the onset and offset of the nocturnal melatonin signal. The second specific aim will determine the relative importance of SCN MT 1 receptor expressing cells in the generation of seasonal responses. Anti-sense ODNs directed against the NT-1 receptor will be infused within the SCN in attempt to block short-day reproductive responses. The goal of the third specific aim is to characterize how time of day, photoperiod, and melatonin regulate AVP peptide levels within the SCN of the Siberian hamster using in vivo microdialysis. Our results will have applications to human circadian disorders including jet lag, sleep disorders, seasonal depression, and infertility.